Earth’s interior- Layers of the earth

Introduction

In this article, we will discuss the earth’s interior and the internal layers of the earth.

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The surface of the earth is an outcome of the processes operating in the interior of the earth. Both exogenic and endogenic forces are constantly shaping the landscape of Earth. It is fascinating to know how scientists have gathered information about the different layers of the Earth.

The radius of the earth is about 6370km. Till now, no one can reach the centre of the earth. Most of our knowledge about the earth’s interior is based on the estimates and inferences. However, the study of seismic waves has contributed immensely to our understanding of the different layers of the earth. For instance, the velocity and the path travelled by the waves provide the authenticity of the physical conditions prevailing inside the earth.

Sources of information about the layers of the earth

Direct sources

• Mining.
• Deep oceanic drilling projects.
• Volcanic eruption.

Indirect sources

• Temperature and pressure increase with the increasing distance from the surface towards the Earth’s interior.
• The density of the material.
• Gravitational Anomaly.
• Seismic activities.

Based on the physical conditions, the earth can be divided into three layers. That are crust, mantle and core. These layers are further subdivided based on their properties.

Layers of the earth

Crust

• It is the uppermost and the thinnest layer of the earth.
• The average thickness of the crust is about 35 km.
• Moreover, the crust can be further divided into the Continental crust and Oceanic crust.

Continental crust

• In terms of structure, composition, density and thickness, the continental crust differs from the oceanic crust.
• It is composed of granitic and andesitic rocks.
• The continental crust is rich in Feldspar mineral.
• Moreover, the density of the continental crust is about 2.6gm/cm3.
• The average thickness is about 40km.
• However, under the mountainous regions, the thickness reaches up to 100 Km.

Oceanic crust

• The oceanic crust is made up of the Basalt.
• It is rich in ferro-magnesia.
• Its density is about 3 gm/cm^3.
• The Conrad discontinuity divides the continental crust and the oceanic crust.

Moho discontinuity

The moho discontinuity separates the crust and mantle.

Mantle

• The mantle extends from the base of the crust at a depth of 2900km.
• It comprises about 80% of the earth’s total volume.
• Mantle contain Iron, magnesium and calcium.
• Because of increasing temperature and pressure inside the earth, it is hotter and denser than the crust.
• At the depth of about 670km, the mantle is divided into upper and the lower mantle.

Upper mantle

• It comprises of Peridotite and Gabbro and Plagioclase minerals.
• The average density of the upper mantle is about 4.5 gm/cm3.
• The average temperature of this layer is about 1100 C ⁰.
• The Repetti Discontinuity separates the upper mantle from the lower mantle.
• Due to Repetti discontinuity, there is a sharp increase in the velocity of the seismic waves.
• The upper mantle is heterogeneous in terms of density and composition.
• The thickness of the uppermost part of this stratum is about 80-100 km.
• The uppermost part of the upper mantle is as rigid as the crust.
• Both crust and upper mantle constitute the Lithosphere.

Asthenosphere

The asthenosphere is that part of the layer of earth which is below the Lithosphere. It extends at a depth of 100 km to 400 km from the lithosphere. Due to the high temperature, this region is partially molten. Here the velocity of seismic wave slowdowns abruptly. This region is called Low-velocity region. Also, it is rich in Peridotite. Moreover, this region is popularly known as the Magma Chamber.

Lower mantle

• The lower mantle extends at a depth of 670 km to 2900km.
• The average density of this region is about 6.5gm/cm³.
• It is composed of Olivine, Plagioclase and Orthoclase minerals.
• Guttenberg Discontinuity separates the lower mantle and the upper core of the earth.

Core

At the depth of 2900 km to 6371 km, lies the core of the earth. Because of metallic composition, its density is nearly twice as the mantle. It comprises of 15% to 16% of the total volume of the earth. The core is divided into two regions called the outer core and the inner core.

Outer core

• It extends between 2900 km to 5150 km.
• The density of the outer core is about 10gm/cm³.
• It mainly consists of Iron and Nickle (about 85%).
• The outer core is always in the molten state.
• Lehman Discontinuity separates the outer core and the inner-core.

Inner core

• The inner-core extends between 5150 km to 6371 km.
• The average density of the inner core is about 13gm/cm3.
• Despite the high temperature, the inner-core is always in the solid-state due to very high pressure prevailing in this region.
• The temperature of the inner core is about 6000 C⁰.

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